This invention relates to a semiconductor photodetector for use, for example, in an opticsl communication system.
A p.sup.+ in.sup.+ type photodiode and a a so-called avalanche photodiode (APD) for allowing a light-induced carrier to be multiplied through the application of a reverse bias to a pn junction are known as a semiconductor photo-device. In order to properly employ these photodiodes as a photo-detector it is required that the concentration gradient and concentration profile in a semiconductor layer be controlled in a proper fashion.
In a planar type APD, for example, a photo-detecting section is so formed as to provide an abrupt junction and guard ring section for preventing edge breakdown is so formed as to provide a graded junction. In this connection it is known that the lower the background carrier concentration level, the greater the curvature radius of a diffusion layer in a planar device and the lower the carrier concentration gradient, then the greater the breakdown voltage of the graded junction. This is disclosed in IEEE Transactions on Electronic Devices, vol. ED-16, No.11, P924.
The carrier concentration of a multiplication layer is normally of the order of 1 to 3.times.10.sup.16 /cm.sup.3. When the concentration gradient of a diffusion layer is reduced below 2.times.10.sup.20 /cm.sup.4, then a breakdown voltage becomes greater abruptly, and does not depend on the curvature radius of the diffusion layer and a fluctuation in the background concentration.
In the conventional InP type APD, however, in order to form an abrupt pn junction at the photo-detecting section, cadmium, for example, was diffused at high temperature for a brief period. In order to form a graded pn junction at the guard ring section, zinc or cadmium, for example, was diffused as a P type impurity into an n type InP multiplication layer at a lower temperature for a long time period or beryllium was ion implanted followed by a heat treatment. In these methods it has been difficult to adequately attain a greater breakdown voltage difference between the guard ring section and the photo-detecting section.
In FIG. 2, the broken line shows a carrier concentration profile of a device obtained when beryllium was ion implanted into an InP Substitute at an acceleration voltage of 150 KeV and dosage of 5.times.10.sup.13 /cm.sup.2 and the resultant structure was annealed under a partial pressure of phosphorus within an ampoule. This carrier concentration profile is disclosed in "Extended Abstracts of the 45th Autumn Meeting (1984) of the Japan Society of Applied Physics," 13a-K-6 (P. 568). In the semiconductor photo-detector fabricated by this method, the carrier concentration gradient is reduced to as small as 0.6 decade/.mu.m in semilogrithmic scale, where it is understood that by "decade" is meant the exponent to the base 10 of the carrier concentration in atom/cm.sup.3. In the neighborhood of 1.times.10.sup.16 /cm.sup.3 in background carrier concentration the concentration gradient became around 1.times.10.sup.20 /cm.sup.4, but with a gradual approximation to a surface the concentration was increased exponentially with the result that the breakdown voltage of the guard ring layer in the device was not adequate.
In a P.sup.+ in.sup.+ type photodiode, in order to obtain a high-speed response characteristic with a depletion layer adequately extended, the carrier concentration of the i region (intrinsic region) is selected to be below 1.times.10.sup.16 cm.sup.-3, normally at a temperature of 20.degree. C. (this temperature is referred as such unless otherwise indicated to the contrary). In order to form an epilayer of high purity whose Carrier concentration level is below 1.times.10.sup.16 cm.sup.-3, a baking step of a growth reactor is necessary over a longer time period for both the vapor phase growth and the liquid phase growth. Furthermore, the selection of high purity material or the purification of the material is necessary for the intended purpose. This is one cause for a rise in production costs. Within the growth reactor used in the epitaxial growth of P.sup.+ and n.sup.+ high concentration layers there exists a residual impurity. It is therefore difficult to properly control the growth of an i layer and thus to perform a continuous growth of a high purity layer and high concentration layer within the same reactor. This constitutes an obstacle to a complex photosemiconductor device, such as an opto-electronic integrated circuit (OEIC). Thus, the epitaxial growth of a high-purity layer of below 1.times.10.sup.16 cm.sup.-3 in carrier concentration in a P.sup.+ in.sup.+ type photodiode poses a problem in relation to the productivity, cost, and manufacturing flexibility.